Methods and apparatus to implement temperature insensitive threshold detection for voltage supervisors

ABSTRACT

Methods, apparatus, and systems are disclosed for voltage supervisors. An example apparatus includes a first switch having a first source, a first drain, and a first gate, a first resistor having a first terminal and a second terminal, the first terminal coupled to the first source and second terminal coupled to the first drain, a second resistor having a third terminal and a fourth terminal, the third terminal coupled to the second terminal, a third resistor having a fifth terminal and a sixth terminal, the fifth terminal coupled to the fourth terminal, a fourth resistor having a seventh terminal and an eighth terminal, the seventh terminal coupled to the sixth terminal, a second switch having a second source, a second drain, and a second gate, the second source coupled to the seventh terminal, and a comparator having an output, the output coupled to the first gate and the second gate.

RELATED APPLICATION

This patent arises from a continuation of U.S. Provisional PatentApplication Ser. No. 62/978,916, which was filed on Feb. 20, 2020. U.S.Patent Provisional Application Ser. No. 62/978,916 is herebyincorporated herein by reference in its entirety. Priority to U.S.Provisional Patent Application Ser. No. 62/978,916 is hereby claimed.

FIELD OF THE DISCLOSURE

This disclosure relates generally to voltage supervisors, and, moreparticularly, to methods and apparatus to implement temperatureinsensitive threshold detection for voltage supervisors.

SUMMARY

An apparatus includes a first switch having a first source terminal, afirst drain terminal, and a first gate terminal. The apparatus includesa first resistor having a first resistor terminal and a second resistorterminal. The first resistor terminal is coupled to the first sourceterminal and the second resistor terminal is coupled to the first drainterminal. The apparatus includes a second resistor having a thirdresistor terminal and a fourth resistor terminal. The third resistorterminal is coupled to the second resistor terminal. The apparatusincludes a third resistor having a fifth resistor terminal and a sixthresistor terminal. The fifth resistor terminal is coupled to the fourthresistor terminal. The apparatus includes a fourth resistor having aseventh resistor terminal and an eighth resistor terminal. The seventhresistor terminal is coupled to the sixth resistor terminal. Theapparatus includes a second switch having a second source terminal, asecond drain terminal, and a second gate terminal. The second sourceterminal is coupled to the seventh resistor terminal. The apparatusincludes a comparator having an output. The output is coupled to thefirst gate terminal and the second gate terminal.

BACKGROUND

Many modern electronic systems (such as mobile phones, laptops,vehicles, televisions, gaming systems, etc.) include multiple powerrails for powering electronic system components and subsystems. Themultiple power rails may be configured to provide component and/orsubsystem isolation or to supply, different supply voltages fordifferent components and/or subsystems, etc. Power supply supervision insuch electronic systems may involve monitoring each of the power railsto determine whether they are operating within desired voltage ranges(i.e., in-regulation). Furthermore, power supply sequencing may berequired in electronic systems to ensure that the power suppliescorresponding to the various power rails are enabled in a proper order.In many existing electronic systems having multiple power rails, powersupply supervision is implemented as a separate system function commonlyreferred to as a supply voltage supervisor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example system that implements differenttypes of voltage supervisors.

FIG. 2 is a schematic illustration of an example voltage supervisor thatimplements a scaling circuit for monitoring supply voltage.

FIG. 3 is an example plot depicting hysteresis implemented by thescaling circuit of FIG. 2.

FIG. 4 illustrates a signal plot depicting threshold voltage variationsversus system temperature of the voltage supervisor of FIG. 2.

FIG. 5 illustrates a signal plot depicting differences between thresholdvoltage variation over temperature of the voltage supervisor of FIG. 2and a voltage supervisor without the scaling circuit.

FIG. 6 illustrates a probability distribution graph depicting thethreshold voltage of the voltage supervisor of FIG. 2 over ninedifferent temperatures.

The same reference numbers are used in the drawings to depict the sameor similar (by function and/or structure) features. The figures are notto scale.

DETAILED DESCRIPTION

As used herein, references to connections (e.g., attached, coupled,connected, and joined) are to be construed in light of the specificationand, when pertinent, the surrounding claim language. Construction ofconnection references in the present application shall be consistentwith the claim language and the context of the specification whichdescribes the purpose for which various elements are connected. As such,connection references do not necessarily infer that two elements aredirectly connected and in fixed relation to each other.

Descriptors first, second, third, etc., are used herein when identifyingmultiple elements or components which may be referred to separately.Unless otherwise specified or understood based on their context of use,such descriptors are not intended to impute any meaning of priority,physical order or arrangement in a list, or ordering in time but aremerely used as labels for referring to multiple elements or componentsseparately for ease of understanding the disclosed examples. In someexamples, the descriptor “first” may be used to refer to an element inthe detailed description, while the same element may be referred to in aclaim with a different descriptor such as “second” or “third.” In suchinstances, it should be understood that such descriptors are used merelyfor ease of referencing multiple elements or components.

Power on reset (POR) devices and under/over voltage lock out (UVLO/OVLO)devices are devices (e.g., systems) that implement supply voltagesupervision. A POR device generates resetting signals whenever thesupply voltage reaches a certain threshold required by a deviceconfigured to receive the supply voltage (e.g., a subsystem). In someexamples, POR devices are utilized by logic control circuits in digitaland analog subsystems of an electronic device. A UVLO device shuts down(e.g., turns off) the device (such as, DC-DC converter, power switch,amplifier, central processing unit (CPU), accelerator, etc.) configuredto receive the supply voltage in response to the supply voltage droppingbelow a certain threshold voltage. An OVLO devices shuts down (e.g.,turns off) the device (such as, DC-DC converter, power switch,amplifier, central processing unit (CPU), accelerator, etc.) intended toreceive the supply voltage when the supply voltage rises above a certainthreshold voltage.

A common factor required by all three voltage supervision aspectsdescribed above is threshold voltage. The threshold voltage is animportant factor in voltage supervision because the accuracy of thethreshold voltage can greatly impact the operation of the deviceintended to receive the supply voltage. For example, a UVLO device isfabricated for monitoring supply voltage to a DC-DC converter, where 1.8volts of supply voltage is used for operation of the DC-DC converter. Insuch an example, if the UVLO device has a threshold voltage that rangesfrom 1.7 volts to 1.8 volts, the UVLO device may fail to shut down theDC-DC converter when the supply voltage drops below 1.8 volts becausethe threshold voltage is 1.7 volts. Failing to shut down the DC-DCconverter can cause physical damage to the components within device,function failure, performance loss, etc., which are all undesirable.Therefore, devices implementing voltage supervision are designed toproduce accurate (e.g., less than 5% tolerance) and precise thresholdvoltages (e.g., a range between 1.95V volts and 2.0 volts) forcomparison to supply voltages.

In some examples, temperature, process, and component variations mayaffect the precision and/or accuracy of the threshold voltagesimplemented and/or generated by the voltage supervisors. In someconventional voltage supervisors, a bandgap reference is utilized tominimize and/or eliminate temperature dependent threshold voltages. Abandgap reference is a temperature independent voltage reference circuitthat generates a fixed voltage regardless of power supply variations,temperature changes, and circuit loading. However, bandgap referencesmay add undesired complexity, latency, and extra costs to the voltagesupervisors.

Therefore, some voltage supervisors do not use bandgap references. Insome examples, the voltage supervisor includes a comparator thatcompares two voltages generated by a bipolar transistor pair, responsiveto a supply voltage. When the two voltages are equal, the comparatortoggles. In examples disclosed herein, a toggling point of thecomparator can be referred to as the threshold voltage. For example,when the supply voltage meets the threshold voltage, the comparatortoggles. In these recent voltage supervisors, the threshold voltage maybe subject to variation (e.g., may change) depending on the temperatureof the bipolar transistors and other components used to design thevoltage supervisor.

In some examples, the threshold voltage is temperature insensitive whentemperature coefficients of a difference in up-scaled base to emittervoltage (ΔVbe) between two transistors (e.g., a difference in base toemitter voltage that is increased based on a resistor ratio) and thebase to emitter voltage (Vbe) of one of the transistors cancel out.However, the threshold voltage will be temperature dependent when thetemperature coefficients of two voltages do not cancel. For example, atvoltages greater than a bandgap voltage (e.g., 1.2 volts, an industrystandard), the base to emitter voltage, Vbe, may have a negativetemperature coefficient that does not match the positive temperaturecoefficient of the up-scaled difference in base to emitter voltagebetween two transistors. In some examples, the two voltages (up-scaledΔVbe and Vbe) only cancel when the threshold value is equal to thebandgap voltage (e.g., approximately 1.2 volts). Therefore, in recentvoltage supervisors, if the threshold voltage is above or below thebandgap voltage, the threshold voltage may be subject to variation whentemperature increases and/or decreases from room temperature (e.g.,normal operating temperature). For example, when the threshold voltageis above the bandgap voltage of the circuit, there is a positivetemperature coefficient (e.g., threshold voltage increases whentemperature increases) and when the threshold voltage is below thebandgap voltage, there is a negative temperature coefficient (e.g., thethreshold voltage decreases when temperature increases). The more thatthe threshold voltage deviates from the bandgap, the larger thetemperature coefficient.

Examples disclosed herein include a scaling circuit, implemented by avoltage supervisor, that can generate a temperature insensitivethreshold voltage above the bandgap voltage. Examples disclosed hereinsolve the problem of when the threshold voltage of the voltagesupervisor is above the bandgap voltage (e.g., 1.2 volts) and thecircuit is subject to temperatures greater than or less than roomtemperature. In examples disclosed herein, if the voltage supervisor isrequired to toggle and/or trip (e.g., providing an output indicative ofthe supply voltage) at a voltage that is above the bandgap voltage, thenthe described approach could do so. The detection circuit disclosedherein causes the voltage supervisor to toggle at a threshold voltagethat is an upscaled version of the bandgap voltage (e.g., a thresholdvoltage that is greater than the bandgap voltage) and includes a verylow temperature coefficient (e.g., a temperature coefficient with atemperature variation less than ±1% over −40° C. to 125° C.).

The example scaling circuit includes a number of resistors that aretuned to scale the bandgap voltage. The number of resistors are tunedbased on the desired threshold voltage. For example, the desiredthreshold voltage is utilized to determine appropriate resistor values,wherein appropriate resistor values are ones that generate temperatureindependent and/or temperature insensitive voltages when the supplyvoltage is at the desired threshold. In other examples, the scalingcircuit includes a number of resistors that are tuned based on desiredhysteresis of two threshold voltages. For example, the scaling circuitimplements hysteresis for a rising threshold voltage and a fallingthreshold voltage in order to avoid oscillation at an output of thevoltage supervisor when the supply voltage is near the threshold voltagewith noise and ripple. The rising threshold voltage is greater than thefalling supply threshold, and the difference between the two thresholdvoltages is the hysteresis. In examples disclosed here, the scalingcircuit ensures that the rising threshold voltage, the falling thresholdvoltage, and the hysteresis are all insensitive to temperaturevariation.

FIG. 1 is a block diagram of an example system 100 that implementsdifferent types of voltage supervisors. The example system 100 includesan example supply voltage generator 102, example digital blocks 104,example analog blocks 106, an example first voltage supervisor 108, anexample second voltage supervisor 110, and an example third voltagesupervisor 112. In the example of FIG. 1, the first voltage supervisor108 is a power-on reset (POR) circuit, the second voltage supervisor 110is an under voltage lockout (UVLO) circuit, and the third voltagesupervisor 112 is an over voltage lockout (OVLO) circuit.

The system 100 may be any type of power operated device, such as acomputer, a telephone, a television, a smart watch, etc. The system 100is powered by the supply voltage generator 102 (which may include apower supply generator such as a battery, direct current (DC) powersupply and/or a regulator, such as a low drop-out (LDO), buck, boost, orbuck-boost regulator). The supply voltage generator 102 is configuredand/or adapted to provide adequate power supply to the digital blocks104 and the analog blocks 106 of the system 100. For example, the supplyvoltage generator 102 turns the digital blocks 104 and the analog blocks106 on and off, responsive to a control signal.

The digital blocks 104 and the analog blocks 106 perform and/or executeoperations for the system 100. For example, the digital blocks 104 andthe analog blocks 106 include a central processing unit (CPU) core andmixed signal arrays of configurable integrated analog and digitalperipherals that make up a system on a chip (SoC). An SoC is anintegrated circuit that integrates all or most components of a computeror other electronic system. In some examples, the digital blocks 104 arecircuit blocks and/or circuitry including state machines, logic gates,flip-flops, microcontrollers, microprocessors, etc. In some examples,the analog blocks 106 are circuit blocks and/or circuitry includingelectrical components such as amplifiers, power switches, etc. Thedigital blocks 104 and the analog blocks 106 are configured to receivesupply voltage from the supply voltage generator 102. In some examples,the digital blocks 104 operate at specific specifications, such as arange of voltages (e.g., minimum and maximum supply voltagerequirements), temperatures, etc. In some examples, the analog blocks106 operate at specific specifications, different or the same as thedigital blocks 104, such as a range of voltages, temperatures, etc. Dueto the specific specifications and requirements of the digital blocks104 and analog blocks 106, the system 100 includes the voltagesupervisors 108, 110, and 112. While voltage supervisors 108, 110 and112 are depicted as three separate circuit blocks and/or separatedcircuitry in FIG. 1, in some examples, these voltage supervisors may beimplemented in fewer (e.g. one) or more circuit blocks and/or circuitryor they may be included in supply voltage generator 102.

The first voltage supervisor 108 is a POR circuit that generates resetsignals whenever power is supplied to a given electrical device (e.g.,the digital blocks 104 and/or the analog blocks 106). For example, thePOR circuit is a circuit that provides a predictable, regulated voltageto the digital blocks 104 and the analog blocks 106 with the initialapplication of power. The first voltage supervisor 108 detects the levelof supply voltage output by the supply voltage generator 102 and trips(e.g., toggles) when the supply voltage exceeds a threshold voltage(Vth).

The first voltage supervisor 108 includes an input terminal and anoutput terminal. The input terminal of the first voltage supervisor 108is coupled to an output terminal of the supply voltage generator 102 ata first node 101. The output terminal of the first voltage supervisor108 is coupled to a first reset terminal (e.g., a circuit block inputterminal) of the digital blocks 104 and a second reset terminal (e.g., acircuit block input terminal) of the analog blocks at a second node 103.

The second voltage supervisor 110 is a UVLO circuit that turns off thepower of an electronic device (e.g., the digital blocks 104 and/or theanalog blocks 106) responsive to the supply voltage decreasing below anoperational value. The second voltage supervisor 110 detects the levelof supply voltage output by the supply voltage generator 102 and trips(e.g., toggles) when the supply voltage decreases below a thresholdvoltage (Vth).

The second voltage supervisor 110 includes an input terminal and anoutput terminal. The input terminal of the second voltage supervisor 110is coupled to the output terminal of the supply voltage generator 102 atthe first node 101. The output terminal of the second voltage supervisor110 is coupled to a first UVLO terminal (e.g., a circuit block inputterminal) of the digital blocks 104 and a second UVLO terminal (e.g., acircuit block input terminal) of the analog blocks at a third node 105.

The third voltage supervisor 112 is an OVLO circuit that turns off thepower of an electronic device (e.g., the digital blocks 104 and/or theanalog blocks 106) responsive to the supply voltage increasing above anoperational value. The third voltage supervisor 112 detects the level ofsupply voltage output by the supply voltage generator 102 and trips(e.g., toggles) when the supply voltage increases above a thresholdvoltage (Vth).

The third voltage supervisor 112 includes an input terminal and anoutput terminal. The input terminal of the third voltage supervisor 112is coupled to the output terminal of the supply voltage generator 102 atthe first node 101. The output terminal of the third voltage supervisor112 is coupled to a first OVLO terminal (e.g., a circuit block inputterminal) of the digital blocks 104 and a second OVLO terminal (e.g., acircuit block input terminal) of the analog blocks at a fourth node 107.

The example voltage supervisors 108, 110, and 112 include an examplefirst scaling circuit 114 a, an example second scaling circuit 114 b, anexample third scaling circuit 114 c, an example first comparison circuit116 a, an example second comparison circuit 116 b, and an example thirdcomparison circuit 116 c. In FIG. 1, the scaling circuits 114 a, 114 b,and 114 c generate a scaled version of the bandgap voltage when thesupply voltage meets the supply threshold voltage. For example, when thesupply threshold voltage is above the bandgap voltage the scalingcircuits 114 a, 114 b, and 114 c scale the bandgap voltage such thatsupply voltage is temperature insensitive when the supply voltagereaches the supply threshold. In this manner, there is no temperaturedrift at the supply threshold voltage and the voltage supervisors 108,110, and 112 accurately toggle the output, regardless of temperature. Insome examples, the amount of scaling of the bandgap voltage is based onthe desired supply threshold voltage, wherein the desired supplythreshold voltage is determined based on the purpose of each voltagesupervisor 108, 110, 112. For example, the first voltage supervisor 108triggers at a desired power-on-reset threshold voltage (Vth POR). Inother examples, the second voltage supervisor 110 triggers at a desiredunder voltage lock out threshold (Vth UVLO). In other examples, thethird voltage supervisor 110 triggers at a desired over voltage lock outthreshold (Vth OVLO). In each example, the scaling circuits 114 a, 114b, and 114 c operate to scale the bandgap voltage to enable a managementof temperature variation that can occur in the system 100, as describedin further detail below in connection with FIG. 2.

In some examples, the scaling circuits 114 a, 114 b, and 114 c includehysteresis. For example, hysteresis is added to the scaling of bandgapvoltage to enable the voltage supervisors 108, 110, 112 to detect adifferent rising threshold voltage than the falling threshold voltage.In some examples, there are noise and ripple at the first node 101 fromthe supply voltage generator 102. If the supply voltage is around (e.g.,approximately at) the threshold voltage, there may be some noise andripple that causes the supply voltage to go up and down, which causesthe voltage supervisors 108, 110, 112 to toggle back and forth. Forexample, if the threshold voltage is 2 volts and the supply voltage isapproximately equivalent to 2 volts, noise and ripple from the supplyvoltage generator 102 can cause the supply voltage to go a little above2 volts (e.g., from a few microvolts to tens of millivolts) and a littlebelow 2 (e.g., from a few microvolts to tens of millivolts) volts forthe period of time that the supply voltage is equivalent to thethreshold voltage. In such examples, the output of the voltagesupervisors 108, 110, 112 may toggle back and forth instead of one timefor a rising supply voltage or one time for a falling supply voltage.The scaling circuits 114 a, 114 b, 114 c therefore include circuitrythat scales the supply voltage in two different ways based on whetherthe supply voltage is rising or falling.

In FIG. 1, the example first comparison circuits 116 a is coupled to thefirst scaling circuit 114 a, the supply voltage generator 102, thedigital blocks 104, and the analog blocks 106, The example secondcomparison circuit 116 b is coupled to the second scaling circuit 114 b,the supply voltage generator 102, the digital blocks 104, and the analogblocks 106. The example third comparison circuit 116 c is coupled to thethird scaling circuit 114 c, the supply voltage generator 102, thedigital blocks 104, and the analog blocks 106. The example comparisoncircuits 116 a, 116 b, and 116 c include voltage supervisor circuitryutilized for toggling the output of the voltage supervisors 108, 110,112. For example, the comparison circuits 116 a, 116 b, and 116 cinclude circuitry, described in further detail below in connection withFIG. 2, that detects the threshold voltage, set by the scaling circuits114 a, 114 b, 114 c, and toggles the output based on the supply voltagemeeting the threshold voltage. In this manner, the comparison circuits116 a, 116 b, 116 c trigger at the desired supply threshold.

The digital blocks 104 and the analog blocks 106 respond to signals atthe respective reset, UVLO, and OVLO terminals. For example, the digitalblocks 104 power down when the second voltage supervisor 110 trips dueto a decrease of supply voltage below the threshold voltage. In otherexamples, the analog blocks 106 power on responsive to the first voltagesupervisor 108 tripping due to a supply voltage meeting the thresholdvoltage. In examples disclosed herein, the first, second, and thirdvoltage supervisors 108, 110, 112 protect the digital blocks 104 andanalog blocks 106 from functional failure, performance loss, devicedamage, etc., caused by the supply voltage being too low or too high. Insome examples, the system 100 includes more than one first voltagesupervisor 108 (e.g., POR circuit), more than one second voltagesupervisor 110 (e.g., UVLO circuit), and/or more than one third voltagesupervisor 112 (e.g., OVLO circuit). For example, the digital blocks 104can include numerous types of state machines, where each state machinerequires different specifications (e.g., different supply/operationalvoltages). In other examples, the analog blocks 106 can include numeroustypes of analog electrical components, where each component includesdifferent specifications (e.g., different supply/operational voltages).

In examples disclosed herein, one or more of the digital blocks 104operates at a supply voltage that is greater than a bandgap voltage(e.g., 1.2 volts). In examples disclosed herein, one or more of theanalog blocks 106 operates at a supply voltage that is greater than thebandgap voltage. In examples disclosed herein, the scaling circuits 114a, 114 b, 114 c and the comparison circuits 116 a, 116 b, 116 c, and/ormore generally, the voltage supervisors 108, 110, and 112, generate anup-scaled version of the bandgap voltage that is used as the thresholdvoltage. In some examples, the scaling circuits 114 a, 114 b, 114 c andthe comparison circuits 116 a, 116 b, 116 c, and/or more generally, thevoltage supervisors 108, 110, and 112, generate two up-scaled versionsof the bandgap voltage, one that is used as the threshold voltage for arising (e.g., increasing) supply voltage and one that is used as thethreshold voltage for a falling (e.g., decreasing) supply voltage. Inthis manner, the scaling circuits 114 a, 114 b, and 114 c generatetemperature insensitive threshold voltages that are reliable intemperature change conditions. An implementation of such scalingcircuits 114 a, 114 b, and 114 c and the comparison circuits 116 a, 116b, 116 c, is described in further detail below in connection with FIG.2.

FIG. 2 is a schematic illustration of an example voltage supervisor 200implementing an example scaling circuit 202 to monitor supply voltage.The voltage supervisor 200 may be implemented by the example firstvoltage supervisor 108 (POR circuit), the example second voltagesupervisor 110 (UVLO circuit), and/or the example third voltagesupervisor 112 (OVLO circuit) of FIG. 1. In some examples, the voltagesupervisor 200 monitors supply voltage output by the supply voltagegenerator 102 of FIG. 1. The voltage supervisor 200 includes an examplefirst switch (M1) 204, an example second switch (M2) 206, an examplefirst transistor (Q1) 208, an example second transistor (Q2) 210, anexample first resistor (R1) 212, an example second resistor (R2) 214, anexample third resistor (R3) 216, an example fourth resistor (R4) 218,example fifth resistors (R5) 220, 222, an example sixth resistor (R6)224, an example comparator 226, and an example logic gate 228. Theexample first switch 204, the example second switch 206, the examplefirst resistor 212, the example second resistor 214, the example thirdresistor 216, and the example fourth resistor 218 make up the examplescaling circuit 202. The example first transistor (Q1) 208, the examplesecond transistor (Q2) 210, the example fifth resistors (R5) 220, 222,the example sixth resistor (R6) 224, the example comparator 226, and theexample logic gate 228 make up an example comparison circuit 236.

The first transistor 208 and the second transistor 210 are implementedby NPN bipolar junction transistors (BJTs). Alternatively, the firsttransistor 208 and the second transistor 210 may be implemented by adifferent type of transistor, such a PNP BJT, a junction gatefield-effect transistor (JFET), a metal-oxide-semiconductor field-effecttransistors (MOSFETs), etc. The first switch 204 is implemented byP-channel metal-oxide-semiconductor field-effect transistors (MOSFETs)(e.g., P-channel silicon MOSFETs, P-channel gallium nitride (GaN)MOSFETs, etc.). Alternatively, the first switch 204 may be implementedby a different type of transistor, such as a bipolar junction transistor(BJT), an N-channel MOSFET, a junction gate field-effect transistor(JFET), etc. The second switch 206 is implemented by an N-channel MOSFET(e.g., N-channel silicon MOSFETs, N-channel gallium nitride (GaN)MOSFETs, etc.). Alternatively, the second switch 206 may be implementedby a different type of transistor, such as a bipolar junction transistor(BJT), a P-channel MOSFET, a junction gate field-effect transistor(JFET), etc.

The second transistor 210 is N times the size of the first transistor208. For example, the first transistor 208 is copied N times to make upsecond transistor 210. In such an example, if N=5, the second transistor210 consists of (e.g., includes, is made up of, etc.) 5 copies of thefirst transistor 208. As used herein, N is indicative of a size ratiothat indicates the difference in area between the first transistor 208and second transistor 210. The size ratio of the transistors 208, 210and/or the size of the second transistor 210 is selected based on devicematching and the area of a chip on which the voltage supervisor 200 isdesigned.

In FIG. 2, the fifth resistors (R5) 220, 222 have substantially equalresistance values. For example, a first one of the fifth resistors 220includes the same value of resistance (in ohms) as a second one of thefifth resistors 222.

In FIG. 2, the logic gate 228 is implemented by an inverter, a NOT gate,etc. Alternatively, the logic gate may be implemented by any type and/ormultiple types of logic gates, such as AND gates, NOR gate, etc.

In FIG. 2, the second resistor 214 is configured and/or adapted to becoupled to the output terminal of the supply voltage generator 102(FIG. 1) at the first node 101. Second resistor 214 is connected betweenthe source and drain terminals of first switch 204. The first resistor212 is coupled in series to the second resistor 214 at a fifth node 201.First resistor 212 is connected between nodes 201 and 203. The thirdresistor 216 is coupled in series to the first resistor 212 at a sixthnode 203. The third resistor 216 is connected between node 203 and thenode 205. The fourth resistor 218 is coupled in series to the thirdresistor 216 at a seventh node 205. The fourth resistor 218 is connectedbetween node 205 and common potential (e.g. ground).

In FIG. 2, a first current terminal (e.g., a source terminal) of thefirst switch (M1) 204 is configured and/or adapted to be coupled to theoutput terminal of the supply voltage generator 102 (FIG. 1) at thefirst node 101. Additionally, the first current terminal of the firstswitch 204 is coupled to the first resistor terminal of the secondresistor 214. The second current terminal (e.g., drain terminal) of thefirst switch 204 is coupled to the second resistor 214 and the firstresistor 212 at the fifth node 201.

In FIG. 2, a first current terminal (e.g., a drain terminal) of thesecond switch (M2) 206 is coupled to the third resistor (R3) 216 and thefourth resistor (R4) 218 at the seventh node 205. The second currentterminal (e.g., a source terminal) of the second switch (M2) 206 iscoupled to ground.

In FIG. 2, the fifth resistors 220, 222 are coupled to the firstresistor 212 and the third resistor 216 at an eighth node 207. As usedherein, the potential at the sixth node 203 and the eighth node 207 isequal. Therefore, the fifth resistors 220, 222 could be coupled to thefirst resistor 212 and the third resistor 216 at the sixth node 203.

In FIG. 2, a collector terminal of the first transistor 208 is coupledto the first one of the fifth resistors 220 at a ninth node 209. Anemitter terminal of the first transistor 208 is coupled is coupled toground. A base terminal (e.g., a control terminal) of the firsttransistor 208 is coupled to the collector terminal of the firsttransistor 208 at a tenth node 211. The base terminal (e.g., controlterminal) of the first transistor 208 may be coupled to the collectorterminal of the first transistor 208 at the ninth node 209.

In FIG. 2, a collector terminal of the second transistor 210 is coupledto the second one of the fifth resistors 222 at an eleventh node 213. Anemitter terminal of the second transistor 210 is coupled to the sixthresistor 224. A base terminal (e.g., control terminal) of the secondtransistor 210 is coupled to the base terminal (e.g., control terminal)of the first transistor 208 and, thus, coupled to the collector terminalof the first transistor 208 at the tenth node 211.

In FIG. 2, the comparator 226 includes a first input 230 (e.g., negativeinput, inverting input, etc.), a second input 232 (e.g., positive input,non-inverting input, etc.), and an output 234. The logic gate 228includes a logic gate input and a logic gate output. The first input 230of the comparator 226 is coupled to the second one of the fifthresistors 222 at the eleventh node 213. The second input 232 of thecomparator 226 is coupled to first one of the fifth resistors 220 andthe collector terminal of the first transistor 208 at the ninth node209. The output 234 of the comparator 226 is coupled to the logic gateinput at a twelfth node 215. The output 234 of the comparator 226 iscoupled to a gate of the first switch 204 and a gate of the secondswitch 206 at the twelfth node 215. In the example of FIG. 2, thepositive supply rail of comparator 226 is connected to node 101 and thenegative supply rail is connected to ground (GND).

In FIG. 2, the logic gate 228 includes a logic gate output. The logicgate output is coupled to (e.g., adapted to be coupled to, configured tobe coupled to, directly coupled to, etc.) a receiving device (e.g., theanalog blocks 106 and/or the digital blocks 104 of FIG. 1).

In some examples, the voltage supervisor 200 operates to enable a firstvoltage (V1) at the ninth node 209 to match a second voltage (V2) at theeleventh node 213 when the supply voltage at the first node 101 is equalto the threshold voltage. Additionally, the example voltage supervisor200 is configured to ensure that there is little to no temperature driftwhen the supply voltage reaches the threshold voltage, becausetemperature drift causes voltage variations within the voltagesupervisor 200 that can cause the first voltage (V1) and the secondvoltage (V2) to match at an incorrect time (e.g., when the supplyvoltage is below the threshold voltage, above the threshold voltage,etc.).

In an example operation, the comparison circuit 236 and/or moregenerally, the voltage supervisor 200 is configured to toggle thecomparator output 234 at a threshold voltage of 2.13 volts. For example,the combination of electrical components (e.g., the switches,transistors, and resistors) configure the first voltage (V1) to matchthe second voltage (V2) when the supply voltage is equal to 2.13 voltsand, thus, toggle the comparator output 234. For example, the size ofthe first transistor (Q1) 208 and the second transistor (Q2) 210, andthe sizes of the first resistor (R1) 212, the second resistor (R2) 214,the third resistor (R3) 216, the fourth resistor (R4) 218, the fifthresistors (R5) 220, 222, and the sixth resistor (R6) 224 are selected toconfigure the voltage supervisor 200 to toggle the comparator output 234at the threshold voltage. In the example operation, the voltage at thefirst node 101 (e.g., the supply voltage, Vdd, etc.) is initially lowresponsive to a supply voltage generator (e.g., the supply voltagegenerator 102 of FIG. 1) not outputting voltage. The first transistor(Q1) 208 and the second transistor (Q2) 210 are turned off (e.g., notconducting current) when the supply voltage is low (e.g., approximatelyzero). The supply voltage begins to increase responsive to the supplyvoltage generator 102 outputting voltage. In some examples, the firsttransistor 208 and the second transistor 210 turn on responsive to thesupply voltage increasing to a minimum threshold base-to-emitter(V_(be)) voltage of the first transistor 208 and second transistor 210.In some examples, gradually, there is some current flowing through thefirst transistor 208 and the second transistor 210 responsive to thefirst transistor 208 and second transistor 210 turning on. Initially,when the supply voltage is equal to the minimum threshold V_(be) of thefirst transistor 208 and second transistor 210, the current flowingthrough the second transistor 210 is greater than a current flowingthrough the first transistor 208. For example, the bigger transistor(e.g., the second transistor 210) sinks more current than the smallertransistor (e.g., the first transistor 208) due to the bigger size ofthe second transistor 210. In response to more current flowing throughthe second transistor 210 than the first transistor 208, the voltagedrop across the second one of the fifth resistors 222 is greater thanthe voltage drop across the first one of the fifth resistors 220. Forexample, a larger current I4 times the resistance of the fifth resistor222 generates a greater voltage respective to the smaller current I3times the resistance of the fifth resistor 220. In this manner, thesecond voltage (V2) at the eleventh node 213 is less than the firstvoltage (V1) at the ninth node 209.

In the example operation, the supply voltage at the first node 101continues to increase. The current across the second one of the fifthresistors 222 (I4) increases in response to the supply voltageincreasing. The voltage drop across the sixth resistor (R6) 224increases in response to the collector current of the second transistor210 (I4) increasing. A difference in base-to-emitter voltage (ΔV_(be))of the first transistor 208 and the second transistor 210 increases asthe collector current of the second transistor 210 (I4) increases. Thedifference in base-to-emitter voltage (ΔV_(be)) of the first transistor208 and the second transistor 210 is utilized to determine the currentthrough the second one of the fifth transistors 222 and, thus, can beutilized to indirectly determine second voltage (V2) at the eleventhnode 213. The difference in base-to-emitter voltage (ΔV_(be)) of thefirst transistor 208 and the second transistor 210 can be determinedutilizing Equation (1) below. In Equation (1), R6 is the resistance inohms of the sixth resistor 224 and I4 is the current in amperes acrossthe second one of the fifth resistors 222.

ΔV _(be) =R6×I4  (1)

In some examples, as ΔV_(be) continues to increase, the secondtransistor 210 reaches a point where a base-to-emitter junction can nolonger compensate the V_(be) difference. The V_(be) of the firsttransistor 208 then increases responsive to the inability of the secondtransistor 210 to compensate the difference in V_(be). When the V_(be)of the first transistor 208 (V_(be1)) increases, the current across thefirst one of the fifth resistors 220 (I3) increases. In this manner, thefirst voltage (V1) at the ninth node 209 increases slower than thesecond voltage (V2) at the eleventh node 213, moving closer to thesecond voltage (V2). When the supply voltage is equal to the thresholdsupply voltage (e.g., 2.13 volts), the current through the firsttransistor 208 (I3) and the current through the second transistor 210(I4) are equal.

In some examples, the current through the second transistor (Q2) 210 isI4 and can be derived from Equation 1 above by solving for I4 or can bedetermined utilizing Equation (2) below. The current through the secondtransistor (Q2) 210 (I4) is derived in order to find the comparatortoggling point (i.e., the supply detection threshold). At the comparatortoggling point, the current through the first transistor (Q1) 208 (e.g.,current I3) is equal to the current I4. In Equation (2), V3 is a thirdvoltage at the eighth node 207, V_(be1) is the base-to-emitter voltageof the first transistor (Q1) 208, and R5 is the resistance (in ohms) ofthe second one of the fifth resistors 222.

$\begin{matrix}{I_{4} = {I_{3} = \frac{V_{3} - V_{be1}}{R5}}} & (2)\end{matrix}$

In the example operation, the third voltage (V3) can also be referred toherein as an intermediate voltage. The third voltage (V3) and/or theintermediate voltage is the result of (e.g., generated by) the scalingcircuit 202. The example scaling circuit 202 implements a resistordivider to scale down the supply voltage. In examples disclosed herein,the scaling circuit 202 is implemented to ensure temperaturecoefficients of the up-scaled ΔV_(be) and V_(be1) are balanced at thesupply threshold. For example, at the supply voltage threshold(V_(dd_th)), temperature coefficients of up-scaled ΔV_(be) and V_(be1)are to cancel, enabling a temperature insensitive detection point. Inany voltage supervisor, the toggling point (e.g., the supply thresholdvoltage) is derived based on making the first voltage (V1) equal to thesecond voltage (V2). In a conventional voltage supervisor (a voltagesupervisor without the scaling circuit 202), the toggling point can bedetermined based on Equation (3) below. In Equation (3), V_(be1) is thebase to emitter voltage of a first transistor (e.g., first transistor208) and ΔV_(be) is a difference between the base-to-emitter voltage ofthe first transistor (e.g., first transistor 208) and a base-to-emittervoltage of a second transistor (e.g., second transistor 210).

$\begin{matrix}{V_{{dd}\;\_\;{th}} = {{\frac{R_{5}}{R_{6}}\Delta V_{be}} + V_{be1}}} & (3)\end{matrix}$

In some examples, the difference between the base-to-emitter voltage ofthe first transistor and the base-to-emitter voltage of the secondtransistor (ΔV_(be)) (e.g., the first transistor 208 and secondtransistor 210) is based on temperature. For example, ΔV_(be) can bedetermined utilizing Equations (4) and (5) below. In Equation (4), V_(T)is indicative of the thermal voltage of the two bipolar transistors(e.g., the first transistor 208 and second transistor 210) and ln(N) isthe natural log of the size ratio of the two bipolar transistors.

ΔV _(be) =V _(T)*ln(N)  (4)

The thermal voltage (V_(T)) of the two bipolar transistors depends onthe absolute temperature (T) of the conventional voltage supervisor indegrees Kelvin. For example, the thermal voltage (V_(T)) can be definedby utilizing Equation (5) below. In Equation (5) below, T is absolutetemperature of the first transistor and second transistor, k isBoltzmann's constant, and q is the elementary charge (e.g., a constant).

$\begin{matrix}{V_{T} = \frac{k*T}{q}} & (5)\end{matrix}$

In general, voltages have a negative temperature coefficient, meaningthat a voltage reduces when temperature increases. Therefore, thetemperature coefficient of V_(be1) for the first transistor (e.g., firsttransistor 208) in diode-configuration (e.g., the first transistorconnected as a diode because the collector is shorted with the base offirst transistor 208) is negative. For example, if the current throughthe first transistor is constant, V_(be1) decreases as temperatureincreases and V_(be1) increases as temperature decreases. The differencein base-to-emitter voltage of the first transistor (e.g., firsttransistor 208) and the base-to-emitter voltage of the second transistor(e.g., second transistor 210) (ΔV_(be)) has a positive temperaturecoefficient, meaning that ΔV_(be) increases when the temperatureincreases. For example, ΔV_(be) is proportional to absolute temperature,as illustrated in Equations (4) and (5) above, and, thus, increases astemperature increases and decreases as temperature decreases. Thepositive temperature coefficient can be designed by choosing the valueof N and the resistance ratio of the fifth resistor (e.g., fifthresistor 222) and the sixth resistor (e.g., sixth resistor 224).

In some examples, temperature coefficients of ΔV_(be) and thebase-to-emitter voltage of the first transistor (e.g., first transistor208) (V_(be1)) cancel when the supply voltage (V_(dd)) is equal to aparticular voltage. In conventional voltage supervisors, this particularvoltage, as illustrated in Equation (3), is the bandgap voltage(V_(BG)). In this manner, in an operation of a conventional voltagesupervisor (one without the scaling circuit 202), the comparator toggleswhen the supply voltage (V_(dd)) is equal to 1.2 volts. The supplyvoltage at 1.2 volts is temperature independent and, thus, will notsignificantly change with temperature. However, in a different operationof a conventional voltage supervisor, the comparator is configured totoggle when the supply voltage (Vdd) is above or below the bandgapvoltage. In such an operation, the threshold voltage may have a positiveor negative temperature coefficient, depending on whether the thresholdvoltage is greater than or less than the bandgap voltage.

For example, in an operation of the conventional voltage supervisor(e.g., the voltage supervisor without the scaling circuit 202) where thethreshold voltage is 2.13 volts (e.g., above V_(BG)), the comparatortoggles responsive to the supply voltage (Vdd) equaling 2.13 volts.However, at 2.13 volts, the temperature increases and so does ΔV_(be),making the temperature coefficient of up-scaled ΔV_(be) more positivethan needed to cancel the negative temperature coefficients of the baseto emitter voltage of the first transistor (V_(be1)). When the positivetemperature coefficient of up-scaled ΔV_(be) is more than the negativetemperature coefficient of V_(be1), the temperature coefficients of twovoltages (e.g., up-scaled ΔV_(be) and V_(be1)) do not cancel. As such,the threshold voltage has a positive temperature coefficient. Thegreater the threshold voltage increases above the bandgap voltage, thegreater the temperature coefficient increases.

The conventional voltage supervisor (e.g., the voltage supervisorwithout the scaling circuit 202) is not ideal in situations where thetoggling point is configured to be above or below the bandgap voltage(e.g., 1.2 volts). As described above, the threshold voltage is subjectto variation when increased above and/or below the bandgap voltage,which can cause inaccurate monitoring of the supply voltage (Vdd).Inaccurate monitoring of the supply voltage (Vdd) can lead to amisrepresentation of the supply voltage (Vdd) and therefore a falseindication that the supply voltage is at a level sufficient to operate aload (e.g., such as the digital blocks 104 and/or analog block 106 ofFIG. 1). Examples disclosed herein and described in further detail belowconfigure the voltage supervisor 200 to accurately represent the supplyvoltage for a threshold that is greater than the bandgap voltage.

In FIG. 2, the example scaling circuit 202 includes a resistor dividerto scale up the bandgap voltage, such that the threshold voltage can beabove the bandgap voltage and remain temperature insensitive. Inexamples described herein, it should be noted that the third voltage(V3) is not bandgap voltage at the comparator toggling point. For simpleexplanation of the resistor divider implemented by the scaling circuit202, the following description excludes the first switch 204, the secondswitch 206, the second resistor 214, and the fourth resistor 218. Forexample, the following description is written such that the first switch204, the second switch 206, the second resistor 214, and the fourthresistor 218 do not exist (that is, resistors 214 and 218 are shorted),and the output of the supply voltage generator 102 (FIG. 1) is coupledto the first resistor 212 (e.g., or that the second resistor 214 andfirst switch 204 are shorted and the fourth resistor 218 and secondswitch 206 are shorted to ground).

In examples disclosed herein, the scaling circuit 202 is utilized tomake the threshold voltage (V_(dd_th)) temperature insensitive. In someexamples, the threshold voltage (V_(dd_th)) is temperature insensitiveby up-scaling a bandgap voltage. For example, when the supply voltageequals the bandgap voltage, the temperature coefficients of up-scaledΔV_(be) and V_(be1) cancel out, enabling the supply threshold voltage tonot be affected by temperature variation. As such, to ensure that thethreshold voltage, which is greater than the bandgap voltage, is notaffected by temperature variation, the scaling circuit 202 determinesthe threshold voltage (V_(dd_th)) from the bandgap voltage equation anda voltage ratio (r). The voltage ratio (r) is utilized to manipulate thesupply detection threshold, such that the voltage ratio (r) up-scalesthe bandgap voltage. Therefore, the first resistor (R1) 212 and thethird resistor (R3) 216 have specific values that are determined basedon the desired toggling point (e.g., V_(dd_th)) of the voltagesupervisor 200. For example, the first resistor (R1) 212 and the thirdresistor (R3) 216 are utilized not only as a resistor divider todown-scale the supply voltage, but also as the voltage ratio (r) toup-scale the bandgap voltage (V_(BG)) to the desired voltage level, asshown in Equation (6) below. For example, Equation (6) below illustratesthe threshold voltage equation, where the variable r is the voltageratio and the variable V_(BG) is the bandgap voltage. Equation (7) belowillustrates the voltage ratio (r) to scale the bandgap voltage. InEquation (7) below, r is the voltage ratio, R1 is the resistance (inohms) of the first resistor 212, and R3 is the resistance (in ohms) ofthe third resistor 216.

$\begin{matrix}{V_{{dd}\;\_\;{th}} = {r*V_{BG}}} & (6) \\{r = {1 + \left( \frac{R1}{R3} \right)}} & (7)\end{matrix}$

In some examples, the voltage ratio (r) is applied to the bandgapvoltage to determine the threshold voltage (V_(dd_th)). Equation (8)below is the bandgap voltage equation. In Equation (8) below, m is theΔV_(be) up-scaling factor (e.g., a constant set by Equation (9) below),ΔV_(be) is the difference in base-to-emitter voltage of the firsttransistor 208 and base-to-emitter voltage of the second transistor 210,and V_(be1) is the base-to-emitter voltage of the first transistor 208.

V _(BG) =m*ΔV _(be) +V _(be1)  (8)

In some examples, with a given size of the sixth resistor 224, thebandgap voltage equation is used to select the size of the fifthresistors 220, 222, as well as the first resistor 212 and the thirdresistor 216, while the ratio of the first resistor 212 and the thirdresistor 216 is defined in Equation (7) above. For example, the variablem can be determined by utilizing a resistor ratio of the first resistor212, the third resistor 216, and the fifth resistors 220, 222, such asthe resistor ratio illustrated in Equation (9) below. In Equation (9)below, R5 is the resistance (in ohms) of the fifth resistors 220, 222,R1 is the resistance (in ohms) of the first resistor 212, R3 is theresistance (in ohms) of the third resistor 216, and R6 is the resistance(in ohms) of the sixth resistor 224 that is initially determined basedon power consumption and die area considerations.

$\begin{matrix}{m = \frac{{R5} + {2*\left( \frac{R1*R3}{{R1} + {R3}} \right)}}{R6}} & (9)\end{matrix}$

In some examples, m can be determined utilizing Equation (9) abovebecause from a small-signal analysis point of view (e.g., the point ofview where the quiescent point of the voltage supervisor 200 is foundand the non-linear elements of the voltage supervisor 200 are linearizedat the quiescent point), the supply voltage is the same as ground.Therefore, the first resistor 212 (R1) and the third resistor 216 (R3)are in parallel, connecting the third voltage (V3) to the small signalground. The small-signal current going through R1 and R3 in parallel istwice of the small-signal current in each R5 at the comparator togglingpoint. During design and configuration of the example voltage supervisor200, the resistor values are selected to ensure that at the desiredthreshold voltage, V_(dd_th) is not affected from a variation intemperature. Based on the above information and the above Equations (6),(7), (8) and (9), the supply threshold voltage (V_(dd_th)) can bedetermined. For example, V_(dd_th) can be determined utilizing Equation(10) below.

$\begin{matrix}{V_{{dd}\;\_\;{th}} = {\left\lbrack {{\frac{\Delta V_{be}}{R6}*\left( {{R5} + {2*\frac{R1*R3}{{R1} + {R3}}}} \right)} + V_{be1}} \right\rbrack*\left( {1 + \frac{R1}{R3}} \right)}} & (10)\end{matrix}$

In some examples, the supply threshold voltage (V_(dd_th)) can besimplified. For example, the supply threshold voltage (V_(dd_th)) can besimplified to the bandgap voltage (V_(BG)) times the ratio (r), asillustrated above in Equation 6.

In FIG. 2, the example scaling circuit 202 enables the detection of twothreshold voltages (V_(dd_th)), where both threshold voltages areup-scaled bandgap voltages and, thus, temperature insensitive. Thefollowing description of the example scaling circuit 202 includes allelectrical components making up the scaling circuit 202, including thefirst switch 204, the second switch 206, the second resistor 214, andthe fourth resistor 218.

The example scaling circuit 202 configures a falling threshold voltageand a rising threshold voltage. In some examples, the falling thresholdvoltage is less than the rising threshold voltage. For example, thecomparator 226 toggles (e.g., changes a comparator output state) whenthe supply voltage rises to 3.04 volts and toggles when the supplyvoltage falls to 2.13 volts. Such a difference in toggling points(ΔV_(dd_th)) is indicative of an amount of hysteresis implemented andconfigured by the scaling circuit 202. The difference in toggling points(e.g., the difference in rising and falling threshold voltages,ΔV_(dd_th)) includes a range of voltage values at which the comparator226 will not toggle (e.g., change comparator output state). For example,any level of supply voltage between 2.13 volts and 3.04 volts will notcause the comparator 226 to toggle.

In some examples, the rising supply threshold and the falling supplythreshold are determined based on the amount of desired hysteresis. Forexample, a manufacture may desire a 10% hysteresis between the risingand falling threshold voltages, a 100 millivolt hysteresis, etc. In someexamples, the rising supply threshold and falling supply threshold aredetermined when the desired hysteresis is selected. For example, the 10%threshold hysteresis is selected and 10% of the threshold voltage(V_(dd i)n) is added to threshold voltage (V_(dd i)n) for the risingthreshold voltage. In this manner, the rising threshold voltage is 10%greater than the falling threshold voltage (V_(dd_th)). In such anexample, the falling threshold voltage is just equivalent to V_(dd_th).

Turning to FIG. 3, an example plot 300 is illustrated to depict thehysteresis ΔV_(dd_th) The example plot 300 includes an example risingsupply threshold line 302 (V_(dd_th_rise)) and an example falling supplythreshold line (V_(dd_th_fall)) 304. In some examples, the rising supplythreshold line (V_(dd_th_rise)) 302 is the threshold voltage for risingsupply voltage (V_(dd)) set by the scaling circuit 202 of FIG. 2. Insome examples, the falling supply threshold line 304 is the thresholdvoltage for falling supply voltage (V_(dd)) set by the scaling circuit202 of FIG. 2. In the plot 300, the x-axis represents the supply voltageV_(dd) at the first node 101 and the y-axis represents the output of thelogic gate 228.

In an example operation of the voltage supervisor 200, as the supplyvoltage (V_(dd)) increases, the output of the logic gate 228 stays lowas depicted in plot 300. In some examples, when the supply voltageV_(dd) meets the rising supply voltage (V_(dd_th_rise)), the output ofthe logic gate 228 goes high as depicted by the rising supply thresholdline (V_(dd_th_rise)) 302. In some examples, the supply voltage V_(dd)begins to decrease as depicted in plot 300. When the supply voltageV_(dd) meets the falling threshold voltage (V_(dd_th_fall)), the outputof the logic gate 228 goes low as depicted by the falling supplythreshold voltage line (V_(dd_th_fall)) 304. The difference between therising supply threshold voltage 302 and falling supply threshold voltage304 is the hysteresis (ΔV_(dd_th)).

Turning back to FIG. 2, the example voltage supervisor 200 implementshysteresis utilizing a feedback loop. For example, the output of thecomparator 226 is fed back to the scaling circuit 202 and received bythe gate terminal of the first switch 204 and the gate terminal of thesecond switch 206. The feedback loop turns the first switch 204 and thesecond switch 206 on and off based on the output of the comparator 226.The first switch 204 and the second switch 206 are not on at the sametime due to the different channel types (e.g., P-channel and N-channel)of the switches. For example, when the comparator output state at theoutput 234 is a high state, the first switch 204, implemented by aP-channel MOSFET, is off because of the positive voltage being appliedto the gate terminal. Additionally, when the output 234 is high, thesecond switch 206, implemented by an N-channel MOSFET, is on due to thepositive voltage applied to the gate terminal. In some examples, whenthe comparator output state at the output 234 is a low state, the secondswitch 206 is off due to the zero V_(gs) applied to the gate and sourceterminals and the first switch 204 is on due to the negative V_(gs)applied to the gate and source terminals.

In the example operation of the voltage supervisor 200, when the supplyvoltage V_(dd) is low and increasing, the comparator output state at theoutput 234 is as high as the supply voltage. As such, the first switch204 turns off (e.g., the first switch 204 is deactivated) and the secondswitch 206 turns on (e.g., the second switch 206 is initiated) as longas the supply voltage is above the threshold of second switch (M2) 206.In some examples, the fourth resistor 218 is shorted responsive to thesecond switch 206 turning on. For example, the fourth resistor 218 doesnot drop a voltage. In some examples, the resistance of the secondresistor 214 is added to the resistance of the first resistor 212responsive to the first switch 204 being turned off and, therefore,open. For example, the total resistance of an upper branch of thescaling circuit 202 (e.g., the first switch 204, the first resistor 212,and the second resistor 214) is the resistance of the first resistor 212plus the resistance of the second resistor 214. In response to the firstswitch 204 being off and the second switch 206 being on, the totalresistance of the lower branch of the scaling circuit 202 (e.g., thesecond switch 206, the third resistor 216, and the fourth resistor 218)is the resistance of the third resistor 216 only.

In some examples, when the comparator 226 output node 234 is high, thetotal resistance of the lower branch of the scaling circuit 202 is theresistance of the R3 216, and the total resistance of the upper branchof the scaling circuit 202 is the resistance of R1 212 plus theresistance of R2 214, the voltage ratio (r) is higher relative to whenthe total resistance of the lower branch including resistances of bothR3 216 and R4 218 and the upper branch including only the resistance ofthe R1 212. For example, the voltage ratio (r) is determined based onthe total resistance of the upper branch over the total resistance ofthe lower branch. In some examples, this voltage ratio (r) sets therising threshold voltage. Therefore, the rising threshold voltage is setresponsive to the first switch 204 opening (e.g., turned off) and thesecond switch 206 turning on. In some examples, when the first switch204 is open and the second switch 206 is on, the voltage ratio(r_(rise)) for the rising supply threshold (V_(dd_th_rise)) can bedetermined utilizing Equation (11) below. In Equation (11) below, R1 isthe resistance (in ohms) of the first resistor 212, R2 is the resistance(in ohms) of the second resistor 214, and R3 is the resistance (in ohms)of the third resistor 216.

$\begin{matrix}{r_{rise} = {\frac{V_{{dd}\;\_\;{th}\;\_\;{rise}}}{V_{BG}} = {1 + \frac{{R1} + {R2}}{R3}}}} & (11)\end{matrix}$

In the example operation of the voltage supervisor 200, when thecomparator 226 output node 234 is low, the first switch 204 turns on(e.g., the first switch 204 is initiated) and the second switch 206turns off (e.g., the second switch 206 is deactivated). For example, theV_(gs) on the P-channel MOSFET 204 becomes more negative, turning it onand the V_(gs) of the N-channel MOSFET 206 also becomes zero, turning itoff. In some examples, the second resistor 214 is shorted responsive tothe first switch 204 turning on. For example, the second resistor 214does not drop a voltage. Therefore, the total resistance of the upperbranch (e.g., the numerator of the voltage ratio) is just the resistanceof the first resistor 212. In some examples, the resistance of thefourth resistor 218 is added to the resistance of the third resistor 216to equal the total resistance of the lower branch (e.g., the denominatorof the voltage ratio) responsive to the second switch 206 being turnedoff. For example, the total resistance of the lower branch of thescaling circuit 202 is the resistance of the third resistor 216 plus theresistance of the fourth resistor 218.

In some examples, when the numerator of the voltage ratio is theresistance of the R1 212 and the denominator of the voltage ratio is theresistance of R3 216 plus the resistance of R4 218, the falling supplythreshold (V_(dd_th_fall)) is set. For example, the voltage ratio(r_(fall)) is lower relative to the voltage ratio (r_(rise)) of Equation(11) above, thus setting a lower threshold voltage than the thresholdvoltage set by the voltage ratio (r_(rise)) of Equation (11) above.Therefore, the falling threshold voltage (V_(dd_th_fall)) is setresponsive to the first switch 204 turning on and the second switch 206turning off. In some examples, when the first switch 204 is on and thesecond switch 206 is off (e.g., open), the voltage ratio (r_(fall)) forthe falling supply threshold (V_(dd_th_fall)) can be determinedutilizing Equation 12 below. In Equation (12) below, R1 is theresistance (in ohms) of the first resistor 212, R3 is the resistance (inohms) of the third resistor 216, and R4 is the resistance (in ohms) ofthe fourth resistor 218.

$\begin{matrix}{r_{fall} = {\frac{V_{{dd}\;\_\;{th}\;\_\;{fall}}}{V_{BG}} = {1 + \frac{R1}{{R3} + {R4}}}}} & (12)\end{matrix}$

In such examples shown by Equations (11) and (12) above, to meet therequirements of the rising threshold voltage (V_(dd_th_rise)) and thefalling threshold voltage (V_(dd_th_fall)), the resistance values of theupper branch and the lower branch are to change based on which thresholdvoltage is to be detected. For example, if the rising threshold voltage(V_(dd_th_rise)) is to be detected, the resistance of the upper branchincreases due to the addition of the second resistor 214 and theresistance of the lower branch decreases due to the subtraction from thefourth resistor 218. Alternatively, if the falling threshold voltage(V_(dd_th_fall)) is to be detected, the resistance of the lower branchincreases due to the addition of the fourth resistor 218 and theresistance of the upper branch decreases due to the shorting of thesecond resistor 214. Therefore, the resistance values of the secondresistor 214 and the fourth resistor 218 are to be carefully selected toensure the rising and falling threshold voltages are correctly set.

In some examples, the resistance values of R2 214 and R4 218 aredetermined based on two factors: one is that the values are to enable(e.g., configure, equal, etc.) the desired hysteresis ΔV_(dd_th), thesecond is the idea that the value of m, determined by Equation 9 above,cannot change. Turning to the second factor used for selecting theresistance values of R2 214 and R4 218, m is the value that is set inorder to ensure the threshold voltage is temperature insensitive.Therefore, the total resistance of upper branch and lower branch inparallel must remain the same in order to keep m unchanged whendetermining resistance values for R2 214 and R4 218. In examplesdisclosed herein, for a rising supply voltage, the total upper branchresistance is the combination of the resistance of R1 212 and theresistance of R2 214. In examples disclosed herein, for a falling supplyvoltage, the total lower branch resistance is the combination of theresistance of R3 216 and the resistance of R4 218.

In some examples, due to the nature of the scaling circuit 202, thesecond resistor (R2) 214 and the fourth resistor (R4) 218 are not usedat the same time. For example, when the supply voltage is rising, thetotal resistance in parallel of the scaling circuit 202 includes thefirst resistor (R1) 212, the second resistor (R2) 214, and the thirdresistor (R3) 216. In other examples, when the supply voltage isfalling, the total resistance in parallel of the scaling circuit 202includes the first resistor (R1) 212, the third resistor (R3) 216, andthe fourth resistor (R4) 218. Therefore, to ensure that the totalresistance in parallel of the scaling circuit 202 is kept constant whenthe supply voltage is rising and when the supply voltage is falling isdetermined utilizing Equation (13) below.

$\begin{matrix}{\frac{R1*\left( {{R3} + {R4}} \right)}{{R1} + \left( {{R3} + {R4}} \right)} = \frac{\left( {{R1} + {R2}} \right)*R3}{\left( {{R1} + {R2}} \right) + {R3}}} & (13)\end{matrix}$

By utilizing Equation (13) above, the resistance of the second resistor214 (R2) and the resistance of the fourth resistor 218 (R4) can bederived. However, the first factor used for selecting the resistancevalues of R2 214 and R4 218 is to be taken into consideration. Forexample, the desired hysteresis ΔV_(dd_th) is used to determine how muchchange in voltage ratio (Δr) is needed and, thus, how much resistanceshould be on R2 214 and R4 218. The change in voltage ratio (Δr), asillustrated in Equation (14) below, is directly proportional to thehysteresis (ΔV_(dd_th)), where the greater the change in Δr, the greaterthe hysteresis. The change in voltage ratio is the ratio applied toup-scale the bandgap voltage for rising threshold voltage to the ratioapplied to up-scale the bandgap voltage for falling threshold voltage.In some examples, to keep m unchanged, R2 214 is linearly related to thechange in voltage ratio (Δr), R2 214 increases linearly. Additionally,R4 218 is nonlinearly related to the change in voltage ratio (Δr). Insome examples, Equation (15) and Equation (16) can be used tosystematically select the resistance values of R2 214 and R4 218. InEquation (15), Δr is the change in voltage ratio, and r is the voltageratio related to the falling threshold (i.e., r=r_(fall)).

$\begin{matrix}{{\Delta r} = {{r_{rise} - r_{fall}} = {\frac{V_{{dd}\;\_\;{th}\;\_\;{rise}} - V_{{dd}\;\_\;{th}\;\_\;{fall}}}{V_{BG}} = \frac{\Delta V_{{dd}\;\_\;{th}}}{V_{BG}}}}} & (14) \\{\frac{R4}{{R3} + {R4}} = \frac{\Delta r}{r*\left( {r - 1 + {\Delta r}} \right)}} & (15) \\{\frac{R2}{R1} = \frac{\Delta r}{\gamma}} & (16)\end{matrix}$

Equation (15) above corresponds to the relationship between theresistance of the third and fourth resistors 216, 218 and the voltageratios for the rising and falling threshold voltages. Equation (16)above corresponds to the relationship between the resistance of thefirst and second resistors 212, 214 and the voltage ratios for therising and falling threshold voltage. The scaling circuit 202 improvesthe efficiency of designing a voltage supervisor 200 because resistorvalues can be determined with a simple calculation rather than aplug-and-chug guessing method. For example, a designer can utilizeEquations (15) and (16) above to determine resistor values of thescaling circuit 202 rather than guessing values, testing them, thenadjusting the values based on the testing results. The latter can takean inefficient amount of time. In some example embodiments, resistors212, 214, 216 and 218 can be implemented using programmable resistor(s)(or variable resistors) that can be programmed/varied in operation orprior to operation by values stored in memory. In other exampleembodiments, resistors 212, 214, 216 and 218 may be external to thescaling circuit 202 and, thereby, determined after manufacturing of thescaling circuit 202.

The example voltage supervisor 200 is an improvement over conventionalvoltage supervisors because the scaling circuit 202 enables a risingthreshold detection point and falling threshold detection point that aretemperature insensitive, even above the bandgap voltage. The voltagesupervisor 200 is an improvement over conventional voltage supervisorsbecause the scaling circuit 202 configures hysteresis to ensure that nooscillation occurs at the output 234 of the comparator 226.

FIG. 4 illustrates signal plots depicting threshold voltage variations(Vth) versus system temperature for the voltage supervisor 200 of FIG.2. The signal plots of FIG. 4 include a first plot 400 a correspondingto the rising threshold voltage variations versus system temperature.The signal plots of FIG. 4 include a second plot 400 b corresponding tothe falling threshold voltage variations versus system temperature. Thesignal plots of FIG. 4 include a third plot 400 c corresponding to thevoltage variations of the hysteresis versus system temperature.

The first plot 400 a includes a first threshold voltage line 402, thesecond plot 400 b includes a second threshold voltage line 404, and thethird plot 400 c includes a hysteresis line 406. In FIG. 4, the firstthreshold voltage line 402 may represent the signal (e.g., voltage) atthe first node 101 of the voltage supervisor 200 of FIG. 2 when thesupply voltage generator 102 outputs the rising threshold voltage. InFIG. 4, the second threshold voltage line 404 may represent the signal(e.g., voltage) at the first node 101 of the voltage supervisor 200 ofFIG. 2 when the supply voltage generator 102 outputs the fallingthreshold voltage. In FIG. 4, the hysteresis line 406 may represent theamount voltage between the rising and falling threshold voltages of thevoltage supervisor 200 of FIG. 2.

In FIG. 4, there are multiple first threshold voltage lines 402,multiple second threshold voltage lines 404, and multiple hysteresislines 406, where one line (for each plot 400 a, 400 b, and 400 c) isindicative of the ideal/nominal process conditions and the other linesrepresent process variations (such as “process corners”). As usedherein, a process corner is an example of a design-of-experimentstechnique that looks at device performance based on device fabricationprocess deviations where each “corner” represents an extreme deviationin one or more process conditions. Process corners represent theextremes of these parameter variations within which a circuit (e.g.,voltage supervisor 200) that has been formed on/over a semiconductorwafer must function correctly. A circuit (e.g., the voltage supervisor200) running on devices fabricated at these process corners may runslower or faster than specified and at lower or higher temperatures andvoltages. In FIG. 4, the ideal/nominal process condition of the voltagesupervisor 200 is represented by the dotted line and is referred to asthe first threshold voltage line 402, the second threshold voltage line404, and the hysteresis line 406. In FIG. 4, a strong process conditionof the voltage supervisor 200 is represented by the triangle lines. InFIG. 4, a weak process condition of the voltage supervisor 200 isrepresented by the dashed lines.

The example first plot 400 a depicts a variation of rising thresholdvoltage as the temperature increases from −40° C. to 125° C. The examplefirst threshold voltage line 402 ranges from about 3.036 volts to 3.04volts for an ideal process condition. In some examples, the risingthreshold voltage is configured at 3.04 volts. Therefore, the first plot400 a demonstrates that there is very little drift of voltage over anextreme range of temperatures with ideal process conditions fordetection of rising threshold voltage.

The example second plot 400 b depicts a variation of falling thresholdvoltage as the temperature increases from −40° C. to 125° C. The examplesecond threshold voltage line 404 ranges from about 2.125 volts to 2.13volts for an ideal process condition. In some examples, the fallingthreshold voltage is configured at 2.13 volts. Therefore, the secondplot 400 b demonstrates that there is very little drift of voltage overan extreme range of temperatures with ideal process conditions fordetection of falling threshold voltage.

The example third plot 400 c depicts a variation of hysteresis as thetemperature increases from −40° C. to 125° C. The example hysteresisline 406 ranges from about 0.916 volts to 0.918 volts for an idealprocess condition. In some examples, the amount of hysteresis isconfigured to be 0.915 volts. Therefore, the third plot 400 cdemonstrates that there is very little drift of hysteresis over anextreme range of temperatures with ideal process conditions.

FIG. 5 illustrates a signal plot 500 depicting threshold voltagevariations versus system temperature for voltage supervisors includingthe scaling circuit 202 and not including the scaling circuit 202. Thesignal plot 500 includes a first threshold voltage line 502 and a secondthreshold voltage line 504. In FIG. 5, the first threshold voltage line502 may represent the signal (e.g., voltage) at the first node 101 ofthe voltage supervisor 200 of FIG. 2 when the supply voltage generator102 outputs the threshold voltage (V_(th)). In FIG. 5, the secondthreshold voltage line 504 may represent the signal (e.g., voltage) ofsupply voltage of a conventional voltage supervisor without the scalingcircuit 202 when a supply generator outputs a threshold voltage. In thesignal plot 500 of FIG. 5, the threshold voltage is approximately 2.1volts for a rising supply voltage (Vdd).

In FIG. 5, there are multiple first threshold voltage lines 502 andmultiple second threshold voltage lines 504, where one line (firstthreshold voltage line 502 and second threshold voltage line 504) isindicative of the ideal/nominal process conditions and the other linesrepresents a process variations (such as “process corners”).

In FIG. 5, the signal plot 500 includes a range of temperatures from−40° C. to 125° C. (horizontal axis) for the voltage supervisor (one setincluding the scaling circuit 202 and the other not including thescaling circuit 202). At a first temperature 506, the voltage supervisor200 and the voltage supervisor without the scaling circuit 202 aresubject to −20° C. (e.g., below the freezing point of water). At thefirst temperature 506, the first threshold voltage line 502 isindicative of approximately 2.13 volts. For example, the thresholdvoltage (V_(dd_th)) of the voltage supervisor 200 is approximately 2.13volts at −20° C. At the first temperature 506, the second thresholdvoltage line 504 is indicative of approximately 1.9 volts. For example,the threshold voltage (V_(dd_th)) of the conventional voltage supervisoris approximately 1.9 volts at −20° C. There is a 230 millivoltdifference at −20° C. between the voltage supervisor 200 and theconventional voltage supervisor without the scaling circuit 202.

At a second temperature 508, the voltage supervisor 200 and theconventional voltage supervisor without the scaling circuit 202 aresubject to 20° C. (e.g., room temperature). At the second temperature508, the first threshold voltage line 502 is indicative of approximately2.13 volts. For example, at 20° C., the threshold voltage (V_(dd_th)) ofthe voltage supervisor 200 is approximately 2.13 volts. At the secondtemperature 508, the second threshold voltage line 504 is indicative ofapproximately 2.1 volts. For example, at 20° C., the threshold voltage(V_(dd_th)) of the voltage supervisor without the scaling circuit 202 isapproximately 2.1 volts, indicating that the second temperature 508 isan ideal temperature, because it does not cause the devices of thevoltage supervisor without the scaling circuit 202 to vary in input andoutput.

At third temperature 510 (100° C., e.g., boiling point of water), thethreshold voltage (V_(dd_th)) of the voltage supervisor 200 isapproximately 2.13 volts, and the threshold voltage (V_(dd_th)) of theconventional voltage supervisor (without the scaling circuit 202) isapproximately 2.37 volts. There is a 240 millivolt difference betweenthe threshold voltages of the voltage supervisor 200 and theconventional voltage supervisor without the scaling circuit 202.

Overall, with the temperature range of −40° C. to 125° C., the thresholdvoltage variation for conventional voltage supervisors without thescaling circuit 202 ranges from approximately 1.84 volts to 2.47 volts.Alternatively, over this same temperature range, the threshold voltagevariation for the voltage supervisor 200 is between approximately 2.10volts and 2.15 volts. Therefore, in examples disclosed herein, thethreshold variation over process and temperature is reduced by a factorof approximately 10 (e.g., 630 millivolt variation reduced to 50millivolt variation) for threshold voltages above the bandgap voltage.

FIG. 6 illustrates a probability distribution graph 600 depicting thethreshold voltage of the voltage supervisor 200 over nine differenttemperatures. In FIG. 6, the probability distribution graph 600 is aMonte-Carlo simulation, where the voltage supervisor 200 is sampledmultiple times with random process variables. For example, theMonte-Carlo simulation randomly varies the fabrication processes of thecomponents in the voltage supervisor 200, runs 200 Monte-Carlosimulations at each temperature point, then distributes the thresholdvoltage of the simulation on the probability distribution graph 600. Thevoltage supervisor 200 utilized during the simulations was configured totoggle at 2.13 volts without hysteresis. As such, the threshold voltage(V_(dd_th)) is configured to equal 2.13 volts.

In FIG. 6, the Monte-Carlo simulation runs at nine differenttemperatures and 200 simulations per temperature. In FIG. 6, theprobability distribution graph 600 illustrates the threshold voltage(V_(dd_th)) of a first simulation temperature MC_0_rise=−40° C., thethreshold voltage (V_(dd_th)) of a second simulation temperatureMC_1_rise=−20° C., the threshold voltage (V_(dd_th)) of a thirdsimulation temperature MC_2_rise=0° C., the threshold voltage(V_(dd_th)) of a fourth simulation temperature MC_3_rise=20° C., thethreshold voltage (V_(dd_th)) of a fifth simulation temperatureMC_4_rise=40° C., the threshold voltage (V_(dd_th)) of a sixthsimulation temperature MC_5_rise=60° C., the threshold voltage(V_(dd_th)) of a seventh simulation temperature MC_6_rise=80° C., thethreshold voltage (V_(dd_th)) of an eighth simulation temperatureMC_7_rise=100° C., and the threshold voltage (V_(dd_th)) of a ninthsimulation temperature MC_8_rise=125° C.

Based on the results of all nine simulation temperatures, the thresholdvoltage (V_(dd_th)) of the voltage supervisor 200 had a mean (e.g.,average) of approximately 2.128 volts and a standard deviation ofapproximately 11.2 millivolts over 1800 simulations. In some examples,such a standard deviation is small relative to the amount of mismatchbetween components randomly configured with different device parametersover the simulations.

Example methods, apparatus, systems, and articles of manufacture toimplement temperature insensitive threshold detection for voltagesupervisors are disclosed herein. Further examples and combinationsthereof include the following:

Example 1 includes an apparatus comprising a first switch having a firstsource terminal, a first drain terminal, and a first gate terminal, afirst resistor having a first resistor terminal and a second resistorterminal, the first resistor terminal coupled to the first sourceterminal and second resistor terminal coupled to the first drainterminal, a second resistor having a third resistor terminal and afourth resistor terminal, the third resistor terminal coupled to thesecond resistor terminal, a third resistor having a fifth resistorterminal and a sixth resistor terminal, the fifth resistor terminalcoupled to the fourth resistor terminal, a fourth resistor having aseventh resistor terminal and an eighth resistor terminal, the seventhresistor terminal coupled to the sixth resistor terminal, a secondswitch having a second source terminal, a second drain terminal, and asecond gate terminal, the second source terminal coupled to the seventhresistor terminal, and a comparator having an output, the output coupledto the first gate terminal and the second gate terminal.

Example 2 includes the apparatus of example 1, wherein the first switchcomprises a P-channel metal-oxide-semiconductor field-effect transistor(MOSFET) and the second switch comprises an N-channel MOSFET.

Example 3 includes the apparatus of example 1, wherein the comparatorincludes a first input and a second input, the apparatus furtherincluding a fifth resistor having a ninth resistor terminal and a tenthresistor terminal, the ninth resistor terminal coupled to the fourthresistor terminal and the fifth resistor terminal at a first node, thetenth resistor terminal coupled to the second input, and a sixthresistor having an eleventh resistor terminal and a twelfth resistorterminal, the eleventh resistor terminal coupled to the ninth resistorterminal, the fourth resistor terminal, and the fifth resistor terminal,the twelfth resistor terminal coupled to the first input.

Example 4 includes the apparatus of example 3, wherein the fifthresistor and the sixth resistor comprise substantially equal resistancevalues.

Example 5 includes the apparatus of example 1, wherein the first sourceterminal is configured to be coupled to a supply voltage generator.

Example 6 includes the apparatus of example 1, wherein the comparatorincludes a first input and a second input, the apparatus furtherincluding a first transistor having a first current terminal, a secondcurrent terminal, and a first control terminal, the first currentterminal coupled to the second input and the first control terminalcoupled to the first current terminal at a first node, and a secondtransistor having a third current terminal, a fourth current terminal,and a second control terminal, the third current terminal coupled to thefirst input and the second control terminal coupled to the first controlterminal.

Example 7 includes the apparatus of example 6, wherein the firsttransistor and the second transistor comprise NPN bipolar junctiontransistors (BJTs).

Example 8 includes the apparatus of example 6, further including a fifthresistor having a ninth resistor terminal, the ninth resistor terminalcoupled to the fourth current terminal.

Example 9 includes the apparatus of example 1, further including a logicgate having a logic gate input and a logic gate output, the logic gateinput coupled to the output of the comparator.

Example 10 includes the apparatus of example 9, wherein the logic gatecomprises an inverter.

Example 11 includes the apparatus of example 1, wherein the comparatorincludes a first input and a second input, the apparatus furtherincluding a fifth resistor having a ninth resistor terminal and a tenthresistor terminal, the ninth resistor terminal coupled to the fourthresistor terminal and the fifth resistor terminal at a first node, thetenth resistor terminal coupled to the second input, a sixth resistorhaving an eleventh resistor terminal and a twelfth resistor terminal,the eleventh resistor terminal coupled to the ninth resistor terminal,the fourth resistor terminal, and the fifth resistor terminal, thetwelfth resistor terminal coupled to the first input, a first transistorhaving a first current terminal, a second current terminal, and a firstcontrol terminal, the first current terminal coupled to the secondinput, the tenth resistor terminal, and the first control terminal atthe first node, a second transistor having a third current terminal, afourth current terminal, and a second control terminal, the thirdcurrent terminal coupled to the first input and the twelfth resistorterminal, and the second control terminal coupled to the first controlterminal, and a seventh resistor having a thirteenth resistor terminalcoupled to the fourth current terminal.

Example 12 includes a system comprising a voltage supervisor including ascaling circuit having a first input terminal and a first outputterminal, the first input terminal configured to be coupled to a supplyvoltage, the scaling circuit configured to generate a first thresholdvoltage and a second threshold voltage, and a comparison circuit havinga second input terminal, a third input terminal, and a second outputterminal, the second input terminal configured to be coupled to thesupply voltage, the third input terminal coupled to the first outputterminal, the comparison circuit configured to toggle the second outputterminal at the first threshold voltage and at the second thresholdvoltage, and a circuit block including a circuit block input terminalcoupled to the second output terminal, the circuit block to operate atthe first threshold voltage, wherein the first threshold voltage isgreater than a bandgap voltage.

Example 13 includes the system of example 12, wherein the voltagesupervisor further includes a first switch having a first sourceterminal, a first drain terminal, and a first gate terminal, a firstresistor having a first resistor terminal and a second resistor terminalcoupled between the first source terminal and the first drain terminal,a second resistor having a third resistor terminal and a fourth resistorterminal, the third resistor terminal coupled to the second resistorterminal, a third resistor having a fifth resistor terminal and a sixthresistor terminal, the fifth resistor terminal coupled to the fourthresistor terminal, a fourth resistor having a seventh resistor terminaland an eighth resistor terminal, the seventh resistor terminal coupledto the sixth resistor terminal, a second switch having a second sourceterminal, a second drain terminal, and a second gate terminal, thesecond source terminal coupled to the seventh resistor terminal, and acomparator having an output, the output coupled to the first gateterminal and the second gate terminal.

Example 14 includes the system of example 13, wherein the first switchcomprises a P-channel metal-oxide-semiconductor field-effect transistor(MOSFET) and the second switch comprises an N-channel MOSFET.

Example 15 includes the system of example 13, wherein the comparatorincludes a negative input and a positive input, the voltage supervisorfurther including a fifth resistor having a ninth resistor terminal anda tenth resistor terminal, the ninth resistor terminal coupled to thefourth resistor terminal and the fifth resistor terminal at a firstnode, the tenth resistor terminal coupled to the positive input, and asixth resistor having an eleventh resistor terminal and a twelfthresistor terminal, the eleventh resistor terminal coupled to the ninthresistor terminal, the fourth resistor terminal, and the fifth resistorterminal, the twelfth resistor terminal coupled to the negative input.

Example 16 includes the system of example 12, further including thescaling circuit comprising a first switch having a first sourceterminal, a first drain terminal, and a first gate terminal, a firstresistor having a first resistor terminal and a second resistor terminalcoupled between the first source terminal and the first drain terminal,a second resistor having a third resistor terminal and a fourth resistorterminal, the third resistor terminal coupled to the second resistorterminal, a third resistor having a fifth resistor terminal and a sixthresistor terminal, the fifth resistor terminal coupled to the fourthresistor terminal, a fourth resistor having a seventh resistor terminaland an eighth resistor terminal, the seventh resistor terminal coupledto the sixth resistor terminal, and a second switch having a secondsource terminal, a second drain terminal, and a second gate terminal,the second source terminal coupled to the seventh resistor terminal, andthe comparison circuit comprising a comparator having a first input, asecond input, and an output, the output coupled to the first gateterminal and the second gate terminal, a fifth resistor having a ninthresistor terminal and a tenth resistor terminal, the ninth resistorterminal coupled to the fourth resistor terminal and the fifth resistorterminal at a first node, the tenth resistor terminal coupled to thesecond input, a sixth resistor having an eleventh resistor terminal anda twelfth resistor terminal, the eleventh resistor terminal coupled tothe ninth resistor terminal, the fourth resistor terminal, and the fifthresistor terminal, the twelfth resistor terminal coupled to the firstinput, a first transistor having a first current terminal, a secondcurrent terminal, and a first control terminal, the first currentterminal coupled to the second input, the tenth resistor terminal, andthe first control terminal at the first node, a second transistor havinga third current terminal, a fourth current terminal, and a secondcontrol terminal, the third current terminal coupled to the first inputand the twelfth resistor terminal, and the second control terminalcoupled to the first control terminal, a seventh resistor having athirteenth resistor terminal coupled to the fourth current terminal, anda logic gate having a logic gate input and a logic gate output, thelogic gate input coupled to the output of the comparator.

Example 17 includes the system of example 12, wherein the circuit blockis an analog block or a digital block.

Example 18 includes a method comprising initiating a first switch anddeactivating a second switch responsive to a comparator output stategoing high, the comparator output state connected to a control terminalof the first switch and a control terminal of the second switch,generating a first voltage ratio and a first voltage corresponding tothe first voltage ratio across a first resistor responsive to the firstswitch initiating and the second switch deactivating, in response to asupply voltage equaling a first threshold voltage, the first thresholdvoltage greater than a bandgap voltage generating a second voltageacross a second resistor that is equal to or proportional to the firstvoltage across the first resistor, comparing the first voltage and thesecond voltage, toggling the comparator output state responsive to thecomparison of the first voltage and the second voltage, and deactivatingthe first switch and initiating the second switch to generate a secondvoltage ratio and a second threshold voltage responsive to the togglingof the comparator output state.

Example 19 includes the method of example 18, wherein the first voltageand the second voltage are equal at the first threshold voltage, thefirst threshold voltage insensitive to temperature variation, the methodfurther including generating a third voltage across the first resistorcorresponding to the second voltage ratio, in response to the supplyvoltage equaling the second threshold voltage, generating a fourthvoltage across the second resistor that is equal to or proportional tothe third voltage across the first resistor, comparing the third voltageto the fourth voltage, the third voltage and fourth voltage to be equalat the second threshold voltage, the second threshold voltageinsensitive to temperature variation, and toggling the comparator outputstate responsive to the comparison of the third voltage to the fourthvoltage.

Example 20 includes the method of example 18, further includinggenerating hysteresis between the first threshold voltage and the secondthreshold voltage, the hysteresis based on the first voltage ratio andthe second voltage ratio generated from the first switch and secondswitch and a plurality of resistors connected to the first switch andsecond switch, the hysteresis to be insensitive to temperaturevariation.

From the foregoing, it will be appreciated that example methods,apparatus and articles of manufacture have been disclosed that generatetemperature insensitive threshold voltages for voltage supervisors thatoperate using threshold voltages above the bandgap voltage. Examplesdisclosed herein include a scaling circuit that up-scales the bandgapvoltage when the supply voltage is equal to the desired thresholdvoltages. Examples disclosed herein implement hysteresis to avoidoscillation at the output of the comparator when the rising supplyvoltage and the falling supply voltage are near the desired thresholdvoltage. Examples disclosed herein configure the hysteresis to beinsensitive to temperature variation and, thus, improve operation of thevoltage supervisor when monitoring supply voltage.

The term “couple” is used throughout the specification. The term maycover connections, communications, or signal paths that enable afunctional relationship consistent with this description. For example,if device A generates a signal to control device B to perform an action,in a first example device A is coupled to device B, or in a secondexample device A is coupled to device B through intervening component Cif intervening component C does not substantially alter the functionalrelationship between device A and device B such that device B iscontrolled by device A via the control signal generated by device A.

A device that is “configured to” perform a task or function may beconfigured (e.g., programmed and/or hardwired) at a time ofmanufacturing by a manufacturer to perform the function and/or may beconfigurable (or re-configurable) by a user after manufacturing toperform the function and/or other additional or alternative functions.The configuring may be through firmware and/or software programming ofthe device, through a construction and/or layout of hardware componentsand interconnections of the device, or a combination thereof.

A circuit or device that is described herein as including certaincomponents may instead be adapted to be coupled to those components toform the described circuitry or device. For example, a structuredescribed as including one or more semiconductor elements (such astransistors), one or more passive elements (such as resistors,capacitors, and/or inductors), and/or one or more sources (such asvoltage and/or current sources) may instead include only thesemiconductor elements within a single physical device (e.g., asemiconductor die and/or integrated circuit (IC) package) and may beadapted to be coupled to at least some of the passive elements and/orthe sources to form the described structure either at a time ofmanufacture or after a time of manufacture, such as by an end-userand/or a third-party.

Circuits described herein are reconfigurable to include the replacedcomponents to provide functionality at least partially similar tofunctionality available prior to the component replacement. Componentsshown as resistors, unless otherwise stated, are generallyrepresentative of any one or more elements coupled in series and/orparallel to provide an amount of impedance represented by the shownresistor. For example, a resistor or capacitor shown and describedherein as a single component may instead be multiple resistors orcapacitors, respectively, coupled in parallel between the same nodes.For example, a resistor or capacitor shown and described herein as asingle component may instead be multiple resistors or capacitors,respectively, coupled in series between the same two nodes as the singleresistor or capacitor.

While particular transistor structures are referred to above, othertransistors or device structures may be used instead. For example,p-type MOSFETs may be used in place of n-type MOSFETs with little or noadditional changes. In addition, other types of transistors (such asbipolar transistors) may be utilized in place of the transistors shown.

As used herein, the terms “terminal”, “node”, “interconnection”, “lead”and “pin” are used interchangeably. Unless specifically stated to thecontrary, these terms are generally used to mean an interconnectionbetween or a terminus of a device element, a circuit element, anintegrated circuit, a device or other electronics or semiconductorcomponent.

Uses of the phrase “ground” in the foregoing description include achassis ground, an Earth ground, a floating ground, a virtual ground, adigital ground, a common ground, and/or any other form of groundconnection applicable to, or suitable for, the teachings of thisdescription. Unless otherwise stated, “about,” “approximately,” or“substantially” preceding a value means +/−10 percent of the statedvalue.

Although certain example methods, apparatus and articles of manufacturehave been disclosed herein, the scope of coverage of this patent is notlimited thereto. On the contrary, this patent covers all methods,apparatus and articles of manufacture fairly falling within the scope ofthe claims of this patent. The following claims are hereby incorporatedinto this Detailed Description by this reference, with each claimstanding on its own as a separate embodiment of the present disclosure.

What is claimed is:
 1. An apparatus comprising: a first switch having afirst current terminal, a second current terminal, and a first controlterminal; a first resistor having a first resistor terminal and a secondresistor terminal, the first resistor terminal coupled to the firstcurrent terminal and the second resistor terminal coupled to the secondcurrent terminal; a second resistor having a third resistor terminal anda fourth resistor terminal, the third resistor terminal coupled to thesecond resistor terminal; a third resistor having a fifth resistorterminal and a sixth resistor terminal, the fifth resistor terminalcoupled to the fourth resistor terminal; a fourth resistor having aseventh resistor terminal and an eighth resistor terminal, the seventhresistor terminal coupled to the sixth resistor terminal; a secondswitch having a third current terminal, a fourth current terminal, and asecond control terminal, the third current terminal coupled to theseventh resistor terminal; and a comparator having an output, the outputcoupled to the first control terminal and the second control terminal.2. The apparatus of claim 1, wherein the first switch comprises aP-channel metal-oxide-semiconductor field-effect transistor (MOSFET) andthe second switch comprises an N-channel MOSFET.
 3. The apparatus ofclaim 1, wherein the comparator includes a first input and a secondinput, the apparatus further including: a fifth resistor having a ninthresistor terminal and a tenth resistor terminal, the ninth resistorterminal coupled to the fourth resistor terminal and the fifth resistorterminal at a first node, the tenth resistor terminal coupled to thesecond input; and a sixth resistor having an eleventh resistor terminaland a twelfth resistor terminal, the eleventh resistor terminal coupledto the ninth resistor terminal, the fourth resistor terminal, and thefifth resistor terminal, the twelfth resistor terminal coupled to thefirst input.
 4. The apparatus of claim 3, wherein the fifth resistor andthe sixth resistor comprise substantially equal resistance values. 5.The apparatus of claim 1, wherein the first current terminal isconfigured to be coupled to a supply voltage generator.
 6. The apparatusof claim 1, wherein the comparator includes a first input and a secondinput, the apparatus further including: a first transistor having afifth current terminal, a sixth current terminal, and a third controlterminal, the fifth current terminal coupled to the second input and thethird control terminal coupled to the fifth current terminal at a firstnode; and a second transistor having a seventh current terminal, aneighth current terminal, and a fourth control terminal, the seventhcurrent terminal coupled to the first input and the fourth controlterminal coupled to the third control terminal.
 7. The apparatus ofclaim 6, wherein the first transistor and the second transistor compriseNPN bipolar junction transistors (BJTs).
 8. The apparatus of claim 6,further including a fifth resistor having a ninth resistor terminal, theninth resistor terminal coupled to the fourth current terminal.
 9. Theapparatus of claim 1, further including a logic gate having a logic gateinput and a logic gate output, the logic gate input coupled to theoutput of the comparator.
 10. The apparatus of claim 9, wherein thelogic gate comprises an inverter.
 11. The apparatus of claim 1, whereinthe comparator includes a first input and a second input, the apparatusfurther including: a fifth resistor having a ninth resistor terminal anda tenth resistor terminal, the ninth resistor terminal coupled to thefourth resistor terminal and the fifth resistor terminal at a firstnode, the tenth resistor terminal coupled to the second input; a sixthresistor having an eleventh resistor terminal and a twelfth resistorterminal, the eleventh resistor terminal coupled to the ninth resistorterminal, the fourth resistor terminal, and the fifth resistor terminal,the twelfth resistor terminal coupled to the first input; a firsttransistor having a fifth current terminal, a sixth current terminal,and a third control terminal, the fifth current terminal coupled to thesecond input, the tenth resistor terminal, and the third controlterminal at the first node; a second transistor having a seventh currentterminal, an eighth current terminal, and a fourth control terminal, theseventh current terminal coupled to the first input and the twelfthresistor terminal, and the fourth control terminal coupled to the thirdcontrol terminal; and a seventh resistor having a thirteenth resistorterminal coupled to the fourth current terminal.
 12. A voltagesupervisor having a first threshold voltage output, the voltagesupervisor including: a scaling circuit having a first input terminaland a first output terminal, the first input terminal configured to becoupled to a supply voltage, the scaling circuit configured to generatea first threshold voltage and a second threshold voltage; and acomparison circuit having a second input terminal, a third inputterminal, and a second output terminal, the second input terminalconfigured to be coupled to the supply voltage, the third input terminalcoupled to the first output terminal, the comparison circuit configuredto toggle the second output terminal at the first threshold voltage andat the second threshold voltage; and wherein the first threshold voltageis greater than a bandgap voltage.
 13. The voltage supervisor of claim12, wherein the voltage supervisor further includes: a first switchhaving a first current terminal, a second current terminal, and a firstcontrol terminal; a first resistor having a first resistor terminal anda second resistor terminal coupled between the first current terminaland the second current terminal; a second resistor having a thirdresistor terminal and a fourth resistor terminal, the third resistorterminal coupled to the second resistor terminal; a third resistorhaving a fifth resistor terminal and a sixth resistor terminal, thefifth resistor terminal coupled to the fourth resistor terminal; afourth resistor having a seventh resistor terminal and an eighthresistor terminal, the seventh resistor terminal coupled to the sixthresistor terminal; a second switch having a third current terminal, afourth current terminal, and a second control terminal, the thirdcurrent terminal coupled to the seventh resistor terminal; and acomparator having an output, the output coupled to the first controlterminal and the second control terminal.
 14. The voltage supervisor ofclaim 13, wherein the first switch comprises a P-channelmetal-oxide-semiconductor field-effect transistor (MOSFET) and thesecond switch comprises an N-channel MOSFET.
 15. The voltage supervisorof claim 13, wherein the comparator includes a negative input and apositive input, the voltage supervisor further including: a fifthresistor having a ninth resistor terminal and a tenth resistor terminal,the ninth resistor terminal coupled to the fourth resistor terminal andthe fifth resistor terminal at a first node, the tenth resistor terminalcoupled to the positive input; and a sixth resistor having an eleventhresistor terminal and a twelfth resistor terminal, the eleventh resistorterminal coupled to the ninth resistor terminal, the fourth resistorterminal, and the fifth resistor terminal, the twelfth resistor terminalcoupled to the negative input.
 16. The voltage supervisor of claim 12,further including: the scaling circuit comprising: a first switch havinga first current terminal, a second current terminal, and a first controlterminal; a first resistor having a first resistor terminal and a secondresistor terminal coupled between the first current terminal and thesecond current terminal; a second resistor having a third resistorterminal and a fourth resistor terminal, the third resistor terminalcoupled to the second resistor terminal; a third resistor having a fifthresistor terminal and a sixth resistor terminal, the fifth resistorterminal coupled to the fourth resistor terminal; a fourth resistorhaving a seventh resistor terminal and an eighth resistor terminal, theseventh resistor terminal coupled to the sixth resistor terminal; and asecond switch having a third current terminal, a fourth currentterminal, and a second control terminal, the third current terminalcoupled to the seventh resistor terminal; and the comparison circuitcomprising: a comparator having a first input, a second input, and anoutput, the output coupled to the first control terminal and the secondcontrol terminal; a fifth resistor having a ninth resistor terminal anda tenth resistor terminal, the ninth resistor terminal coupled to thefourth resistor terminal and the fifth resistor terminal at a firstnode, the tenth resistor terminal coupled to the second input; a sixthresistor having an eleventh resistor terminal and a twelfth resistorterminal, the eleventh resistor terminal coupled to the ninth resistorterminal, the fourth resistor terminal, and the fifth resistor terminal,the twelfth resistor terminal coupled to the first input; a firsttransistor having a first current terminal, a second current terminal,and a first control terminal, the first current terminal coupled to thesecond input, the tenth resistor terminal, and the first controlterminal at the first node; a second transistor having a third currentterminal, a fourth current terminal, and a second control terminal, thethird current terminal coupled to the first input and the twelfthresistor terminal, and the second control terminal coupled to the firstcontrol terminal; a seventh resistor having a thirteenth resistorterminal coupled to the fourth current terminal; and a logic gate havinga logic gate input and a logic gate output, the logic gate input coupledto the output of the comparator.
 17. The voltage supervisor of claim 12,wherein an analog block or a digital block is to operate at the firstthreshold voltage.
 18. A method comprising: initiating a first switchand deactivating a second switch responsive to a comparator output stategoing high, the comparator output state connected to a control terminalof the first switch and a control terminal of the second switch;generating a first voltage ratio and a first voltage corresponding tothe first voltage ratio across a first resistor responsive to the firstswitch initiating and the second switch deactivating; in response to asupply voltage equaling a first threshold voltage, the first thresholdvoltage greater than a bandgap voltage: generating a second voltageacross a second resistor that is equal to or proportional to the firstvoltage across the first resistor; comparing the first voltage and thesecond voltage; toggling the comparator output state responsive to thecomparison of the first voltage and the second voltage; and deactivatingthe first switch and initiating the second switch to generate a secondvoltage ratio and a second threshold voltage responsive to the togglingof the comparator output state.
 19. The method of claim 18, wherein thefirst voltage and the second voltage are equal at the first thresholdvoltage, the first threshold voltage insensitive to temperaturevariation, the method further including: generating a third voltageacross the first resistor corresponding to the second voltage ratio; inresponse to the supply voltage equaling the second threshold voltage;generating a fourth voltage across the second resistor that is equal toor proportional to the third voltage across the first resistor;comparing the third voltage to the fourth voltage, the third voltage andfourth voltage to be equal at the second threshold voltage, the secondthreshold voltage insensitive to temperature variation; and toggling thecomparator output state responsive to the comparison of the thirdvoltage to the fourth voltage.
 20. The method of claim 18, furtherincluding generating hysteresis between the first threshold voltage andthe second threshold voltage, the hysteresis based on the first voltageratio and the second voltage ratio generated from the first switch andsecond switch and a plurality of resistors connected to the first switchand second switch, the hysteresis to be insensitive to temperaturevariation.